The present invention pertains to a magenta inkjet ink and in particular to an aqueous based magenta inkjet ink comprising a mixture of certain soluble magenta colorants. The invention also pertains to an ink set comprising this magenta ink.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
Inkjet printers are equipped with an ink set which, for full color printing, typically comprises a cyan, magenta and yellow ink (CMY). An ink set will also commonly comprise a black ink (CMYK). Colorants for the inks can be soluble in the ink vehicle (dyes) or insoluble (pigments).
The ink set should provide printed images having good color characteristics, such as correct hue and high chroma. Preferably, the ink set will achieve these favorable characteristics on a range of media including plain paper as well as specialty media such as transparency film and coated paper. Also, preferably, the hard copy output is reasonably lightfast.
Individual inks should generally exhibit good crusting resistance, good stability, proper viscosity, proper surface tension, good color-to-color bleed alleviation, rapid dry time, consumer-safety, and low strike-through.
Various magenta, yellow and cyan dyes, and CYM dye sets, have been disclosed for plain paper and photo paper printing. Most of the attention though is directed to selection of the magenta colorant.
Probably the most commonly mentioned magenta dye is Reactive Red 180 which can be used alone or in combination with Acid Red 52 to achieve a more violet shade. The combination of dyes is adjusted to give the desired balance of hue, chroma and lightfastness. Reactive dyes such as RR180 are known to hydrolyze in the aqueous ink vehicle causing the pH to drift and producing sulfate ions which cause problems with ink reliability. The reactive dyes can be hydrolyzed and purified before use but this adds an additional expensive processing step to the ink production.
As noted in U.S. Pat. No. 6,641,257, magenta is typically the colorant most vulnerable to light. It is desirable for a dye to have the best possible fade resistance. The '257 reference provides lightfastness evaluation of a number of magenta dyes of which Acid Red 37, Acid Red 289, Direct Red 75, Direct Red 212, Reactive Red 141, Reactive Red 23, Reactive Red 195 are rated highest, and more lightfast than Reactive Red 180.
A magenta ink with excellent lighffastness comprising a mixture of Magenta 377 and Acid 52 is disclosed in U.S. Pat. No. 5,772,742. Hydrolyzed Reactive Red 23 magenta colorant is disclosed in U.S. Pat. No. 6,458,195 and described as having high lightfastness.
Other known magenta colorants include Direct Red 227 as disclosed, for example, in U.S. Pat. No. 5,143,547 as part of a CYM ink set. A mixture of magenta anthrapyridone dye and Reactive Red 23, Reactive Red 24, Reactive Red 31, Reactive Red 120, Reactive Red 180, Reactive Red 241, Acid Red 35, Acid Red 52, Acid Red 249, Acid Red 289, Acid Red 388, Direct Red 227 or CAS No. 153204-88-7 is disclosed in U.S. Pat. No. 6,706,102. A mixture of Acid Red 92 and Reactive Red 180, Direct Red 75, or Reactive Red 159 is disclosed in U.S. Pat. No. 6,793,722.
As inkjet printheads deliver smaller and smaller drops for improved printing resolution, colorant loadings must increase to compensate for the lower amounts of ink applied. However, increasing dye concentration in an ink can cause problems with reliability. For example, oftentimes “latency” or “decap”, one aspect of reliability, is negatively impacted (reduced) by increased dye concentrations. The decap (latency) time refers to the amount of time inkjet nozzles can remain uncapped and idle, and still fire without missing or misdirected drops. Preferably, to avoid printing defects, an ink will preferably exhibit favorable (long) decap times even at high dye loadings.
There is still a need for improved colorants and/or colorant selection for magenta dye-based inks and in particular for dyes that can be used reliability at high loadings and provide good hue, chroma, optical density and fade resistance.